Onhan siellä wikipediassa näköjään selitetty: Normaalilla veriarvolla voi vielä olla puutetta. Nykyiset normit veriarvoille saattaa olla turhan matalat. Japanissa on nostettu minimiarvoa reilusti arvosta 200 pg/mL arvoon 550 pg/mL. Seerumin homokysteiinin ja metyylimalonihapon tasoa pidetään ilmeisesti luotettavampina mittareina B12-puutteelle kuin veren B12-pitoisuutta. 10% henkilöistä, joilla veriarvo on välillä 200–400 pg/mL, kärsii puutteesta homokysteiinin ja metyylimalonihapon tason perusteella.
Serum B12 levels are often low in B12 deficiency, but if other features of B12 deficiency are present with normal B12 then further investigation is warranted. One possible explanation for normal B12 levels in B12 deficiency is antibody interference in people with high titres of intrinsic factor antibody. Some researchers propose that the current standard norms of vitamin B12 levels are too low. In Japan, the lowest acceptable level for vitamin B12 in blood has been raised from about 200 pg/mL (145 pM) to 550 pg/mL (400 pM).
Serum vitamin B12 tests results are in pg/mL (picograms/millilitre) or pmol/L (picomoles/litre). The laboratory reference ranges for these units are similar, since the molecular weight of B12 is approximately 1000, the difference between mL and L. Thus: 550 pg/mL = 400 pmol/L.
Serum homocysteine and methylmalonic acid levels are considered more reliable indicators of B12 deficiency than the concentration of B12 in blood. The levels of these substances are high in B12 deficiency and can be helpful if the diagnosis is unclear. Approximately 10% of patients with vitamin B12 levels between 200–400pg/ml will have a vitamin B12 deficiency on the basis of elevated levels of homocysteine and methylmalonic acid.
Routine monitoring of methylmalonic acid levels in urine is an option for people who may not be getting enough dietary B12, as a rise in methylmalonic acid levels may be an early indication of deficiency.
If nervous system damage is suspected, B12 analysis in cerebrospinal fluid is possible, though such an invasive test should be considered only if blood testing is inconclusive.
The Schilling test has been largely supplanted by tests for antiparietal cell and intrinsic factor antibodies.
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